# 550 5.1.1: bad destination mailbox address URL: https://iblockspam.com/smtp-errors/550-5-1-1 Last updated: 2026-10-09 Code: 550 5.1.1 (permanent failure, the message bounced) Provider: Standard (RFC) The recipient's mailbox doesn't exist at that domain. It's usually a typo or an old address, so the message can't be delivered as addressed. What the bounce says: ``` 550 5.1.1 The email account that you tried to reach does not exist. Please double-check the recipient's email address for typos or unnecessary spaces. ``` Gmail's text, as listed in Google's SMTP errors reference. RFC 3463 defines X.1.1 as "Bad destination mailbox address": the mailbox specified in the address does not exist, meaning the part left of the "@" is invalid. It is only useful as a permanent failure, which is why it nearly always comes with 550. Microsoft's documentation for 550 5.1.1 through 5.1.20 says the wrong email address is the most common cause of 5.1.x errors. It lists others: a stale entry in the sender's Outlook Auto-Complete list after a mailbox moved, a recipient's forwarding rule that sends to an invalid address, a compromised sender account, and stale MX records on the recipient's domain that point to an old mail server. Compare the address in the bounce with the one you meant to use. If they differ, the recipient probably has a forwarding rule pointing somewhere broken. If you never sent the original message, the bounce may be backscatter: a spammer used your address as the sender, and the bounce came to you. ## Common causes - A typo in the part of the address before the @. - The person left, or the mailbox was deleted or renamed. - A stale Auto-Complete entry in the sender's mail app. - A forwarding rule on the recipient's side sends to an invalid address. - The recipient domain's MX record points to an old server that no longer has the mailbox. ## How to fix it (sender) - Check the spelling and confirm the address with the recipient by another channel. - Delete the address from your mail app's Auto-Complete list and type it fresh. - If you didn't send the original message, treat the bounce as backscatter and check your account wasn't misused. ## How to fix it (recipient or admin) - Confirm the mailbox exists and is licensed, and that the address or alias is spelled correctly. - Check forwarding rules and mail flow rules that copy messages to other addresses. - Make sure your MX records point to your current mail service only. **Q: Can 550 5.1.1 happen if the address is correct?** Yes, occasionally. The mailbox may have been removed, the recipient's forwarding rule may point to a broken address, or their domain's MX record may point to an old server. Ask the recipient to check with their email admin. **Q: I got a 550 5.1.1 bounce for a message I never sent. Why?** That's usually backscatter: a spammer forged your address as the sender, and the receiving server bounced the message to you. Check your Sent folder; if it's clean, change your password as a precaution and make sure your domain has SPF, DKIM and DMARC. **Q: Is 550 5.1.1 a temporary error?** No. RFC 3463 says X.1.1 is only useful for permanent failures. Resending to the same address will fail again. Sources: - RFC 3463: Enhanced Mail System Status Codes: https://www.rfc-editor.org/rfc/rfc3463 - Fix NDR error code 550 5.1.1 through 550 5.1.20 in Exchange Online (Microsoft Learn): https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/ndr/fix-error-code-550-5-1-1-through-5-1-20-in-exchange-online - Gmail SMTP errors and codes (Google Workspace Admin Help): https://knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes --- Source: iBlockSpam, https://iblockspam.com/smtp-errors/550-5-1-1 (this is a Markdown copy; the HTML page is canonical). Quote with attribution and a link. Site index for AI assistants: https://iblockspam.com/llms.txt